This invention relates to the art of insulating compositions useful for electric machines and electric apparatus. More particularly, it concerns electric insulating coating compositions capable of preventing degradation of the insulation resistance caused by the environment or atmosphere in which they are used. A means for rapidly recovering decreased insulation resistance is contemplated and will be described hereinbelow.
Insulation resistance of electric machines and apparatus installed in factory areas or areas of contaminated atmosphere or near the seashore decreases gradually during their operation over extended periods of time. This decrease is due to adsorption, deposition thereon or accumulation thereon of gas, vapor mist, dust or moisture or the like, ultimately resulting in large fault such as short circuit and burn out. Such a tendency to decreased resistance is promoted by severe contamination of the atmosphere by sulfurous gas from a concentration of factories and cities in seaside districts. An increase in the transmission voltage and recent trend to compactness of electric machines and apparatus also contributes to the problem.
As mentioned above, degradation of electric insulating materials is also caused by absorption of moisture, but in such a case decrease in insulation resistance is larger in terms of surface resistance than in volume resistance as shown by graphs shown in FIGS. 1 and 2 of the drawing.
In this case, absorption of moisture greatly decreases the surface resistivity of insulating materials and the presence of an electrolyte on the surface of such insulating material rapidly decreases insulation resistance by forming a film of water. Various electrolytes contained in the atmosphere in the form of gas, vapor, mist and dust are thus adsorbed or deposit or accumulate on the surface of the insulating material. For example, a decrease in the insulation resistance caused by salt which occurs frequently in seaside districts, is created by mist or dust containing electrolytes such as sodium chloride and the like in sea water. Furthermore, decrease in the insulation resistance is caused by such water soluble ions as SO.sub.4.sup.2.sup.-, Cl.sup.-, NO.sub.3, Mg.sup.2.sup.+, K, Na, Ca.sup.2.sup.+, NH.sub.4.sup.-, etc., contained in mist or dust in the contaminated atmosphere. Table 1 below shows some typical examples of water soluble ions contained in dust generated by steel manufacturing plants and electric power plants.
Table 1 ______________________________________ Content of Water Soluble Ions in Dust Accumulated Water soluble ions SO.sub.4.sup.2.sup.- Cl.sup.- NO.sub.3.sup.- Location Where dust was sampled ______________________________________ Steel manufacturing plant A 11.0% 3.04% 0.05% Steel manufacturing plant B 18.5% 1.01% 0.42% Electric power plant 12.3% 0.59% -- ______________________________________
Dusts were collected at ground level and the water-soluble ion content thereof was measured. Principal identified ingredients were aluminum silicate, clay, cement dust and iron oxide. The quantity of the latter was not measured.
Conditions which accelerate a decrease in the insulation resistance and insulation breakdown caused by electrolytes are as follows:
1. Deterioration is particularly accelerated at humidites above the saturated humidity i.e. 75% in the case of NaCl and 81% in the case of (NH.sub.4).sub.2 SO.sub.4 but does not hardly ever occur at a humidity of less than 30%. PA1 2. Deterioration is related to such chemical characteristics as moisture absorptiveness and reactive radicals of the insulation but not always related to electrical characteristics thereof. PA1 3. It varies dependent upon the applied voltage but under definite conditions it is enhanced with increase in the applied voltage. PA1 1. Decrease the humidity of the atmosphere to less than 30% in order to prevent formation of water films. PA1 2. Use dust proof or totally enclosed construction to prevent deposition of electrolytes. PA1 3. Remove electrolytically dissociated ions to prevent formation of electroconductive films. Measures 1 and 2 are practically used but they are not applicable to all insulating materials or components requiring insulation. On the other hand, surface, treatment to carry out measure 3 is relatively simple and advantageous. PA1 1. Strong electrolyte type resin PA1 2. Weak electrolyte type resin
As described above, although the surface condition of insulators are scarcely affected by electrolytes deposited thereon from the atmosphere of low humidity (less than 30%), as the humidity increases the moisture will deposit or diffuse in the surface of insulators to form molecular films of water. When electrolytes are present in the water film, anions and cations having a degree of freedom corresponding to the transference number of ions will be formed in the water film, thus rendering it conductive so as to decrease the insulation resistance of the insulating material. Further, applied voltage therefore causes transfer of metals due to leakage current, local over voltage, electric discharge, tracking, carbonization and other undesirable phenomena which will result in burn out of the insulation material.
The process of degrading the insulation resistance of the insulating material caused by salt hazard is as follows: Salt components (for example, sodium chloride) deposited on the surface of insulators in the form of mist or dust are dissociated by the water film deposited or absorbed from the atmosphere of high humidity to form electroconductive anions and cations. More particularly:
under electrolyte high humidity ions Na Cl Na.sup.+Cl.sup.- (nonconductive) under low (conductive) humidity
Under the influence of applied voltage these anions and cations migrate toward cathode and anode electrodes, respectively, through an aqueous solution or water film to carry a charge thus effecting electric conduction. In other words, a leakage current results to deteriorate insulating strength.
At the same time, the water also undergoes electrolytic dissociation (H.sub.2 O H.sup.+ + OH.sup.-). However, as the electrolytic dissociation constant of water is 1.8 .times. 10.sup..sup.-6 (at 25.degree.C.) and the resistivity is 6.3 .times. 10.sup.8 ohm-cm (at 25.degree.C.) such dissociation does not affect electric resistance so that it is considered that ions that impart conductivity to the water film are electrolyte compositions contained therein.
In this manner, deterioration of electric insulators is attributable to ions formed by the dissociation of electrolytes contained in moisture deposited or adsorbed in the surface of the insulator.
Accordingly, as a means for preventing deterioration or burning of insulators, the following measures may be used: